1. Field of the Invention
The present invention relates to semiconductor devices having micro-area (width) electrodes on nitride semiconductor layers, and particularly to high-current-driven electronic devices, such as laser diodes, high-power LEDs, FETs, and high-frequency devices. Such semiconductor devices comprise, for example, group III-V nitride semiconductors, such as GaN, AlN, and InN, including their mixed crystals AlGaN, InGaN, and AlInGa.
2. Discussion of the Related Art
Nitride semiconductor devices emit light having a wide range of wavelengths from a relatively short-wavelength ultraviolet region to a visible light region including that of red light, and are widely used for semiconductor laser diodes (LDs) and light emitting diodes (LEDs). The nitride semiconductor devices have been developed to increase miniaturization, increase lifetime, and enhance power. These devices have been principally used in electronic devices such as personal computers and DVDs, medical equipment, processing machinery, and light sources for optical fiber communication.
A typical nitride semiconductor device has a multilayer structure including a buffer layer, an n-type contact layer, a crack-free layer, an n-type cladding layer, an n-type light guide layer, an active layer, a p-type electron confinement layer, a p-type light guide layer, a p-type cladding layer, and a p-type contact layer, in that order, on a sapphire substrate. For an LED, it is not necessary to provide the light guide layers or the like. The multilayer structure is provided with n and p electrodes where the active layer is energized to emit light.
In the electrodes, portions establishing an ohmic contact between the electrodes and respective semiconductor layers play a key role, and are generally formed of a high-work-function metal element or an alloy in a single-layer or multilayer form. Some metals can establish ohmic contact with a semiconductor layer by only depositing the metals on the semiconductor layer. For example, a Pd/Pt/Au multilayer film may be used as a p electrode. For an electrode material which does not easily form an ohmic contact with the semiconductor layer only by deposition, heat treatment allows the electrode material to establish an ohmic contact. For example, a Ni/Au multilayer film can be alloyed to be a transparent p electrode functioning as an ohmic electrode by heat treatment.
The n and p electrodes each have a pad electrode for bonding a wire. If the substrate is non-conductive, pad electrodes are provided on both the n and p electrodes because the n and p electrodes are on the same side of the substrate. Since the ohmic contact can easily be formed on an n-type semiconductor layer, the ohmic electrode on an n electrode may be used as a lead wire bonding electrode. A metallized layer may be formed on the n electrodes, instead of wires, so that the device can be mounted face down.
In addition, an insulating layer is provided between the n and p electrodes. The insulating layer may be formed of a single-layer or multilayer oxide film. In an LD, the insulating layer is also used as a functional film having an additional function, such as a current restriction film for controlling a current-injection region or a reflective film provided on the surface of a resonator.
These nitride semiconductor devices are disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2000-299528A.
However, the surface of the above-described Ni/Au electrode is liable to be damaged by heat treatment. Consequently, the resistance at the interface between the Ni/Au electrode and the pad electrode may be increased. If an insulating layer is in contact with the Ni/Au electrode, the insulating layer is degraded by heat thereby reducing adhesion to the electrode. The Pd/Pt/Au electrode does not readily cause such a problem because the Pd/Pt/Au electrode does not require heat treatment. However, an increase in the device temperature during operation may degrade the characteristics of the electrode due to heat, consequently increasing the operating voltage disadvantageously. In addition, a thin electrode or a wide electrode degrades adhesion or mechanical strength, thus causing a problem during high-current operation.